1. Field of the Invention
The present invention relates to a method and apparatus for microlithographic exposure, and more particularly, to the application of such a method in a lithographic projection apparatus.
2. Discussion of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a “lens”. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatuses are described in International Patent Applications WO98/28665 and WO98/40791, incorporated herein by reference.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire reticle pattern onto the target portion in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatus—which is commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, incorporated herein by reference.
In one form of microlithography, a mask defining features is illuminated with radiation from an effective source having an intensity distribution at a pupil plane corresponding to a particular illumination mode. An image of the illuminated mask is projected onto a resist-coated semiconductor wafer.
Problems with the prior art include that in the semiconductor manufacturing industry there is increasing demand for ever-smaller features and increased density of features. In other words the critical dimensions (CDs) are rapidly decreasing and are becoming very close to the theoretical resolution limit of state-of-the-art exposure tools such as steppers and scanners as described above. One solution to this problem is to upgrade the optics of the machine or indeed replace the entire machine. A second possibility is to use masks which include so-called “assisting features”. These are features smaller than the resolution limit of the exposure tool so that they will not print on the wafer, but their presence near features to be imaged produces diffraction effects which can improve contrast and sharpen fine features. A third possibility is to use complementary Phase Shift Masks where the definition of features such as lines and spaces is established by correspondingly phase shifting the electric field amplitude 180 degrees (rather than by correspondingly modulating the amplitude of the electric field as is the case in commonly used binary chromium masks). This has the effect that the energy of the light diffracted at the mask pattern is angularly distributed in such a manner that image contrast and depth of focus are improved for imaging of lines and spaces at resolution limit. However, none of these methods is entirely satisfactory and they can also prove expensive.